Most users of data communications services access data communications networks (e.g. the Internet) using dial-up connections established through the Public Switched Telephone Network (PSTN). The PSTN is still substantially an analog communications network, designed to transmit sounds in the audible range of the human voice.
Digital data is transported across the PSTN by converting the data into an analog signal that is transmitted by varying, or modulating, the frequency, phase, amplitude or other characteristic of a carrier signal. The analog signal is then transmitted over a standard telephone line (referred to as a "local loop") using a power amplifier (e.g. a line driver), that operates to amplify the power of the analog signal.
For high speed data transmission, the use of multi-carrier (e.g., Discrete Multitone (DMT)) modulation schemes in Digital Subscriber Line (DSL) modems is well known. In DMT systems, an input bit stream is first serial-to-parallel converted. The parallel output is then grouped into N groups of bits corresponding to the number of bits per symbol. Portions of bits are allocated to each DMT carrier or sub-channel. The power transmitted over each sub-channel is preferably approximately the same.
In typical multi-carrier modulation systems, the modulation output approximates a normal distribution, with the result that the peak-to-Root Mean Squared (RMS) ratio of the output analog signal is relatively high. Because of this high ratio, the line driver must have high supply voltages (+Vc and -Vc) in order to adequately transmit the occasional high signal peaks without "clipping" or other distortion. However, such high supply voltages result in substantial power dissipation in the line driver. In fact, in a typical Digital Subscriber Line (DSL) modem, the efficiency of the line driver may be as low as 10%.
The efficiency of the line driver can be improved by dynamically changing the level of the supply voltages between a high level (+/-VH) and a low level (+/-VL) in response to changes in the level of the input analog signal. The goal of this strategy is to minimize the voltages supplied to the line diver when the level of the input signal is at a low (approximately RMS) level. Most of the time, therefore, the line driver will be supplied by the low level (+/-VL) voltage. When a signal peak must be transmitted, the supply voltages are switched to their respective high levels (+/-VH) to minimize clipping or other distortion of the signal peak.
U.S. Pat. No. 6,028,486, which issued to Andre on Feb. 22, 2000, teaches a system in which a dual supply amplifier is connected to a high voltage power supply and a low voltage power supply. A switch is provided to enable the dual supply amplifier to operate on the basis of a selected one of the high voltage and lower voltage power supplies. In one embodiment, a pair of amplifier circuits are provided, each of which is supplied by a respective one of the high and low voltage power supplies. The switch is then connected between respective outputs of each of the amplifier circuits and the output of the dual supply amplifier, and operates to connect the output of a selected one of the amplifier circuits to the dual supply amplifier output. In another embodiment, a single amplifier circuit is provided and the switch is connected between the high and low power supplies and the single amplifier. In this case, the switch operates to selectively connect one of the high and low power supplies to the amplifier.
In each of the above embodiments, the switch is controlled by a control signal which is generated on the basis of a comparison between the input signal level and a predetermined threshold. As a result, when the input analog signal is at a low (RMS) value, the control signal operates the switch so that the dual supply amplifier is powered by the low voltage power supply. However, when a signal peak arrives at the dual supply amplifier, the control signal operates the switch so that the dual supply amplifier is powered by the high voltage power supply.
A disadvantage of known systems, as exemplified by the system of U.S. Pat. No. 6,028,486, is that two supply rails (one each for positive and negative voltages) must be used to connect each power supply to the line driver. In a dual supply amplifier system, this means that four supply rails are needed. Typically, a line card (e.g. such as may be used at a central office) will be provided with multiple modems, each having at least one respective line driver. In order to minimize crosstalk between the modems, separate supply rails must be furnished for each line driver. Supplying four separate supply rails to each line driver consumes limited space on the line card, which in turn reduces the number of modems that can be located on a single card, and also tends to complicate the design of the card.
Accordingly, a dual supply line driver which can be conveniently fabricated on a line card having multiple modems remains highly desirable.